Which of the following motors are best for the rolling mills?

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Examoogle · Accepted Answer

D.C. motors

Explanation: DC motors, particularly DC series or compound motors, are preferred for rolling mills because they provide high starting torque, excellent speed regulation, and the ability to handle the sudden, severe load variations inherent in metal rolling processes.

Working Principle: In a rolling mill, the motor must handle sudden impact loads when the metal billet enters the rolls, causing a rapid dip in speed. DC motors respond to this with a self-regulating characteristic where a drop in speed results in a corresponding increase in torque, allowing the motor to maintain stability. Speed control is easily achieved via armature voltage or field current control, providing the precision required for rolling different metal gauges.

Key Formulas: $T_e = k \Phi I_a$ — Electromagnetic torque produced by a DC motor; $N = \frac{V - I_a R_a}{k \Phi}$ — Speed characteristic of a DC motor

Key Points: High starting torque is required to overcome the inertia of the mill rolls.; Dynamic braking is easily implemented in DC drives for rapid deceleration.; Constant power characteristics are often required for finished rolling passes.

Advantages: Excellent speed control range; High overload capacity; Rapid response to control signals

Disadvantages: Requires frequent maintenance due to brushes and commutator; Higher initial cost compared to AC induction motors; Requires power electronics converters (Rectifiers)

Applications: Steel rolling mills; Paper mills; Traction systems

Additional Info: While modern systems are moving toward AC drives with VFD (Variable Frequency Drives), DC motors remain the traditional standard for heavy-duty, high-torque rolling applications.. Squirrel cage induction motors are generally unsuitable for rolling mills because they lack the high starting torque and the fine speed control required under variable load conditions.

Answer: D is correct — DC motors provide the high starting torque and precise, wide-range speed control necessary for the heavy-duty variable loading conditions of rolling mills.

Exam Tip: Always relate the motor's mechanical load requirements to its torque-speed curve; for rolling mills, think 'High Starting Torque' and 'Load Regulation'.

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